1. Field of the Invention
The present invention relates to a planetary gear system, and more particularly to a planetary gear system wherein an internal gear has a circular-arc-shaped tooth profile formed by pins or a combination of pins and rollers, while the tooth profile of each external gear has a trochoidal tooth profile, a circular-arc-shaped tooth, or the like constituted by epitrochoidal parallel curves, each of the external gears is swingably rotated by the rotation of an eccentric member fitted with the external gear, the internal and external gears are made to internally mesh with each other, thereby to output the rotation by decelerating or accelerating the input rotation.
2. Description of the Related Art
Hitherto, various reduction gears using internally meshing planetary reduction gears have been proposed. Amongst these reduction gears, there is a well-known planetary reduction gear which is known as a "Cyclo Reduction Gear" (registered trademark). This reduction gear is arranged as follows: The internal gear has a circular-arc-shaped tooth profile formed by pins or a combination of pins and rollers, while the external gears have a trochoidal tooth profile formed by epitrochoidal parallel curves. Internal pins or a combination of internal pins and internal rollers are fitted in the external gear with a play, each of the external gears swingably rotated by the rotation of an eccentric member fitted to the external gear. The internal and external gears are made to internally mesh with each other to output the rotation by reducing the input rotation. Since this "Cyclo Reduction Gear" permits transmission of a large torque and its reduction ratio is large, it is used as various types of reduction mechanism.
Referring to FIGS. 6 and 7, an example of the known "Cyclo Reduction Gear" will be described below.
FIG. 6 is a cross-sectional view illustrating an example of the known planetary reduction gear, while FIG. 7 is a cross-sectional view taken along the line VII--VII of FIG. 6.
In this example, although the rotation of an input rotating shaft 1 is obtained as the reduced rotation of an output shaft 2, an arrangement may be alternately provided such that the output shaft 2 is fixed, and the reduced rotation is obtained from the internal gear.
Three eccentric members 3 are provided on the input rotating shaft 1 in an individually divided state. A key way 4 is formed in hollow portions of the eccentric members 3, and a key 4A is fittingly inserted into the key way 4. External gears 5 are respectively fitted around the eccentric members 3 via rollers 6. This external gear 5 has around an outer periphery thereof outer teeth 7 constituted by a trochoidal tooth profile. Furthermore, an internal gear 8 which also serves as an outer casing is held stationary in this illustrated example. The internal gear 8 has a circular-arc-shaped tooth profile constituted by external pins 9 internally meshing with the external gears 5. Each of the external gears 5 is provided with internal pin holes 10 into which internal pins 11 are respectively fitted with a play. An internal roller 12 is fitted with a play around an outer periphery of each of these internal pins 11, one end of the pin 11 being closely fitted with an internal pin retaining flange 13. This internal pin retaining flange 13 is formed integrally with the output shaft 2.
In the above-described known internally-meshing-type planetary reduction gear having the three external gears 5 and the three eccentric members 3, the eccentric members 3 are formed as a split type, and the respective eccentric members are disposed on a shaft by being offset 120.degree. from each other.
However, with such a planetary gear system described above, the following problems are encounted.
Although the greater the number of the eccentric members and the external gears, the more advantageous in transmitting the torque, the installation work becomes complicated on the other hand.
In addition, since the arrangement is such that separate eccentric members are installed directly on the the input rotating shaft by means of a key or the like, it takes a long period of time and experienced skills in improving the installation accuracy. Moreover, there is the disadvantage that the eccentric members become out of phase during operation, with the result that engagement between the external and internal gears becomes unsmooth, and backlash is produced, enlarging the play.